This invention relates to a shifting device for a lawn mower and the like and more particularly to an improved shifting device for a hydraulic transmission wherein there is a wider range of neutral movement provided so as to avoid the likelihood of improper or premature shifting into a drive mode.
Many types of vehicles are operated by hydrostatic transmissions because these devices offer very compact and effective operation. With a hydrostatic transmission, there is provided a fluid pump that is driven by the internal combustion engine and which drives a fluid motor for powering the vehicle. In many forms of application, there is also incorporated a power takeoff that is driven by the engine for driving an accessory such as a lawn mower or the like. As has been noted, the use of a hydrostatic transmission has a number of advantages for this purpose. Normally, the hydrostatic transmission is operated by a shift lever that is moveable from a neutral position to a forward position or to a reverse position. The degree of movement from the neutral position to either of the driving positions is effective to increase the speed of movement in each direction. Hence, such devices are extremely simple and advantageous.
These devices have, however, some disadvantages and these disadvantages may be understood by reference to FIG. 1 wherein the relationship between the movement of the shift lever and the operation of the hydrostatic transmission is depicted. The movement of the shift lever is shown on the ordinate while the transmission operation is shown on the abscissa. It should be noted that movement of the shift lever generally affects operation of the transmission in a linear fashion. Of course, there is a certain range wherein the transmission does not generate sufficient power so as to operate the vehicle and this is indicated by the parallel dot-dash lines indicating a lull. This is equivalent to movement of the shift lever through the distance N1. However, it should be noted that the operation is relatively sensitive and any misadjustment in the transmission control mechanism or inadvertent operation of the shift lever by the operator is likely to cause movement of the vehicle which could be undesirable.
It is, therefore, a principal object of this invention to provide an improved shifting device for a hydrostatic transmission.
It is a further object of this invention to provide a shifting device for a hydrostatic transmission wherein the linkage adjustment is not as critical and inadvertent movement of the shift lever is not likely to cause unexpected movement of the powered vehicle.
In accordance with one feature of the invention, the linkage system interconnecting the transmission with the shift lever can be designed so as to provide an area wherein movement of the shift lever does not effect automatic operation of the transmission. Although such an arrangement is effective, there is only a certain amount of tolerable lost motion or free play in the shift mechanism.
It is, therefore, a still further object of this invention to provide an improved transmission control mechanism for a hydrostatic transmission that includes a hydraulic arrangement for limiting the power buildup in at least one direction so as to create a dwell before the transmission will start to operate the vehicle even though the shift lever has been moved toward the operating position.